Synthesis of heteroatom-substituted carbene complexes of Rhodium(I) and Iridium(I) for application in 1-octene hydroformylation

Abstract

In this project, different mono-heteroatom stabilized carbene complexes of rhodium(I) and iridium(I) were prepared as catalyst precursors for the hydroformylation of the 1-octene as a model reaction. The variation of the electronic properties of the carbene ligands were achieved by utilising either electrophilic, π-acceptor Fischer carbenes or σ-donor, basic 1,2,3-triazol-5-ylidenes.The modulation of molecular catalyst electronic and steric properties arguably play the greatest role in optimisation of (homogeneous) catalyst performance. Rhodium(I) Fischer carbene complexes were prepared using a transmetalation method from the group 6 transition metal precursor, [W(CO)5{C(OEt)(p-DMA)}]. The formation of the Rh–Cbond is confirmed by the presence of the carbene carbon atom resonance as a downfield doublet in the corresponding 13C NMR spectrum. Aminolysis was confirmed by an upfield shift of the carbene doublet, corroborating a decrease electronegativity of the carbene complex 2(70%). Furthermore, a decrease in π-character of this bond is substantiated by SC XRD molecular structure where an increase in the Rh-C bond length of complex 2is observed, in comparison to 1. The labile bidentate 1,5-cyclooctadiene (cod) co-ligand is replaced by a pair of π-acidic carbonyl ligands to afford 3with a 90% yield, circumstantiated by the appearance of a pair of doublets at 186.9 ppm and 184.2 ppm. To determine the donor strength of the NN-dimethylaniline ligand, the TEP of 3 was calculated to be 2043 cm-1. The triazolylidene complexes were prepared from the metalation of the corresponding triazolium salts. The preparation of the N3-alkylated 1,2,3-triazolium salt, L2 (63%) proceeded via the “click” reaction, and subsequent alkylation at position 3. The 1,3-diarylated 1,2,3-triazolium salt L3 (69%)was accessed by the copper-free 1,3-dipolar cycloaddition of a 1,3-diaza-2-azoniaallene salt with an aryl-alkyne. Both were deprotonated independently in the presence of [Rh(cod)Cl]2, resulting in rhodium(I) 1,2,3-triazol-5-ylidene complexes, 5 (48%) and 9 (62%) from L2and L3respectively. Once again, the 13C NMR was employed to confirm metalation by the appearance of a downfield doublet signal for 5and 9, representing the respective carbene carbon atoms. The upfield carbene carbon resonances are in stark contrast to the low field chemical shifts observed for the electrophilic withdrawing Fischer carbene carbons. Moreover, a longer Rh-C bond was observed in the molecular crystal structures of 5 (2.029(4) Å) and 9 (2.053(4) Å), compared to the 2.006(16) Å observed for 2. This is indicative of negligible π-character between Rh–C bonds of 5 and 9 Complexes 5 and 9 were subjected to ligand exchange reactions resulting in dicarbonyl complexes 6 and 10 respectively. The presence of the carbonyl ligands allowed for the measurement of TEP values from the FT-IR measurements of the carbonyl stretching frequencies. The TEP values of 6 (2047 cm-1) and 10 (2049 cm-1) confirmed the superior donor ability of the 1,2,3-triazol-5-ylidene ligands compared to the Fischer carbene ligand. The salts L2 and L3 also underwent direct metalation with AgO2, affording the isolated intermediates S2 (52%) and S3 (43%), before transmetalation to [Ir(cod)Cl]2 resulting in the corresponding complexes 7 (63%) and 11 (67%). The successful transmetalation was signalled by the disappearance of a doublet of doublets signal in the 13C NMR spectra, and the appearance of new characteristic carbene carbon atom singlets at ca.173 ppm in both cases. Following ligand exchange of the coordinated cod ligands with carbonyls, new complexes 8 (from 7) and12 (from 11) could be isolated. Analogous with what was observed for their rhodium(I) counterparts, deshielding of the carbene chemical shifts as a consequence of the π-accepting character of the carbonyl co-ligands was observed. The click reaction was employed to obtain the 4,4’-bis(1,2,3-triazole) (L4(24%)), followed by alkylation (L5(69 %)) and coordination to rhodium. The envisaged chelated complex(14(42%))and an unanticipated μ2-bridged complex (13(49%)) were isolated The differing coupling constants displayed in the NMR spectra of these complexes are indicative of a difference in coordination to the metal centre. To corroborate this, the SC XRD structure of 13 was obtained. It was aimed to prepare a ‘mixed’ biscarbene complex containing both a σ-donor and π-acceptor carbene ligand to exploit the ‘push-pull’ electronic effects of the combination of the two different types of carbene ligands. Attempts to isolate a complex bearing the electron withdrawing Fischer carbene ligand and the electron donating 1,2,3-triazol-5-ylidene proved unsuccessful. A range of transmetalation reactions were performed, including transmetalation of a Fischer carbene ligand to a rhodium(I) 1,2,3-triazol-5-ylidene complex and thetransmetalation of a 1,2,3-triazol-5-ylidene ligand to a rhodium(I) Fischer carbene complex. The modulation of molecular catalyst electronic and steric properties play a big role in the (homogeneous) catalyst performance. The isolated rhodium and iridium complexes were screened for activity in hydroformylation reactions, and the most active were the rhodium Fischer carbene complexes. These complexes displayed conversions of up to >99%, with TOFs ranging between 386 h-1and 543 h-1.The bulkier rhodium 1,2,3-triazol-5-ylidene complexes were significantly more regioselective under the same conditions employed for the rhodium Fischer carbene complexes, with n/iso ratios of up to 2.67 . The iridium complexes displayed limited activity with TOFs below 45 h-1under the same conditions as their rhodium counterparts, consolidating the considerable role of the metal centre